This application claims the benefit of Japanese Application No. 2002-014286 filed Jan. 23, 2002.
The present invention relates to a method of measuring the phase offset of an FID (free induction decay) signal, a method of measuring the phase offset of SE (spin echo)/STE (stimulated echo) signals, an MR (magnetic resonance) imaging method, and an MRI (magnetic resonance imaging) apparatus, and more particularly to a method of measuring the phase offset of an FID signal caused by static magnetic field inhomogeneity, a method of measuring the phase offset of SE/STE signals (SE signal and STE signal) caused by static magnetic field inhomogeneity, an MR imaging method in which the phase offset of the FID signal and the phase offset of the SE/STE signals caused by static magnetic field inhomogeneity are corrected, and an MRI apparatus capable of conducting such methods.
Japanese Patent No. 2,898,329 discloses an MR imaging method comprising:
(1) repeatedly conducting data collection in an SSFP (steady state free precession) state with a successively varying amount of phase encoding to acquire data ƒxcexd(0) for individual views xcexd that together fill a k-space;
(2) repeatedly conducting data collection in the SSFP state with the successively varying amount of phase encoding and with an RF phase alternated by 180xc2x0 to acquire data ƒxcexd(1) for individual views xcexd that together fill the k-space;
(3) generating data Axcexd by addition processing or subtraction processing on ƒxcexd(0) and ƒxcexd(1) as given by:
Axcexd=0.5xc3x97Fxcexd(0)+0.5xc3x97Fxcexd(1)
or
Axcexd=0.5xc3x97Fxcexd(0)xe2x88x920.5xc3x97Fxcexd(1); and
(4) reconstructing an image from the resulting data Axcexd.
The data collected in the SSFP state as in the MR imaging method above contain both components of FID signals and components of SE/STE signals.
It is known that when an image is produced from data collected in the SSFP state as in the MR imaging method disclosed in Japanese Patent No. 2,898,329, static magnetic field inhomogeneity, if any, gives rise to band artifacts in the image.
The generation of band artifacts is caused by phase offsets in the FID signals and SE/STE signals due to static magnetic field inhomogeneity, which results in mutual interference between the FID and SE/STE signals.
It is therefore an object of the present invention is to provide a method of measuring the phase offset of an FID signal caused by static magnetic field inhomogeneity, a method of measuring the phase offset of SE/STE signals caused by static magnetic field inhomogeneity, an MR imaging method in which the phase offset of the FID signal and the phase offset of the SE/STE signals caused by static magnetic field inhomogeneity are corrected, and an MRI apparatus capable of conducting such methods.
In accordance with a first aspect, the present invention provides a method of measuring the phase offset of an FID signal characterized in comprising: adding a crusher for resetting the phase of SE/STE signals to a pulse sequence that is repeated for conducting data collection in an SSFP state, and omitting a phase encoding axis pulse therefrom; collecting phase-offset-measurement data in the SSFP state by repeating the resulting pulse sequence; and measuring the phase offset of the FID signal from the phase-offset-measurement data obtained.
In the method of measuring the phase offset of an FID signal of the first aspect, since the phase of the SE/STE signals is reset by adding a crusher, the phase-offset-measurement data obtained exhibits the phase offset of the FID signal component. Therefore, the phase offset of the FID signal can be measured from the phase-offset-measurement data obtained.
In accordance with a second aspect, the present invention provides the method of measuring the phase offset of an FID signal having the aforementioned configuration, characterized in that: said crusher for resetting the phase of SE/STE signals is a gradient pulse that is applied to at least one of a phase encoding axis and a read axis at a time after a data collection period.
In the method of measuring the phase offset of an FID signal of the second aspect, since a gradient pulse is applied to at least one of a phase encoding axis and a read axis at a time after a data collection period, the phase of the SE/STE signals can be reset without affecting the phase of the FID signal component.
In accordance with a third aspect, the present invention provides a method of measuring the phase offset of SE/STE signals characterized in comprising: adding a crusher for resetting the phase of an FID signal to a pulse sequence that is repeated for conducting data collection in an SSFP state, and omitting a phase encoding axis pulse therefrom; collecting phase-offset-measurement data in the SSFP state by repeating the resulting pulse sequence; and measuring the phase offset of the SE/STE signals from the phase-offset-measurement data obtained.
In the method of measuring the phase offset of FID signal of the third aspect, since the phase of the FID signal is reset by adding a crusher, the phase-offset-measurement data obtained exhibits the phase offset of the SE/STE signal component. Therefore, the phase offset of the SE/STE signals can be measured from the phase-offset-measurement data obtained.
In accordance with a fourth aspect, the present invention provides the method of measuring the phase offset of SE/STE signals having the aforementioned configuration, characterized in that: said crusher for resetting the phase of an FID signal is a gradient pulse that is applied to at least one of a phase encoding axis and a read axis at a time after an RF pulse and before a data collection period.
In the method of measuring the phase offset of an FID signal of the fourth aspect, since a gradient pulse is applied to at least one of a phase encoding axis and a read axis at a time after an RF pulse and before a data collection period, the phase of the FID signal can be reset without affecting the phase of the SE/STE signal component.
In accordance with a fifth aspect, the present invention provides an MR imaging method characterized in comprising: adjusting the phase of an RF pulse in a pulse sequence that is repeated for conducting data collection in an SSFP state to correct the phase offsets of an FID signal and SE/STE signals; collecting imaging data in the SSFP state by repeating the resulting pulse sequence; and producing an image from the imaging data obtained.
In the MR imaging method of the fifth aspect, since the zeroth-order phase offsets of the FID signal and SE/STE signals can be corrected by adjusting the phase of the RF pulse, band artifacts caused by static magnetic field inhomogeneity can be reduced.
In accordance with a sixth aspect, the present invention provides an MR imaging method characterized in comprising: adding a correction pulse for correcting the phase offsets of an FID signal and SE/STE signals to a pulse sequence that is repeated for conducting data collection in an SSFP state; collecting imaging data in the SSFP state by repeating the resulting pulse sequence; and producing an image from the imaging data obtained.
In the MR imaging method of the sixth aspect, since the first-order phase offsets of the FID signal and SE/STE signals can be corrected by adding a correction pulse, band artifacts caused by static magnetic field inhomogeneity can be reduced.
In accordance with a seventh aspect, the present invention provides an MR imaging method characterized in comprising: adjusting the phase of an RF pulse in a pulse sequence that is repeated for conducting data collection in an SSFP state and adding a correction pulse for correcting the phase offsets of an FID signal and SE/STE signals to correct the phase offsets of the FID signal and SE/STE signals; collecting imaging data in the SSFP state by repeating the resulting pulse sequence; and producing an image from the imaging data obtained.
In the MR imaging method of the seventh aspect, since the zero-th order and first-order phase offsets of the FID signal and SE/STE signals can be corrected by adjusting the phase of the RF pulse and adding a correction pulse, band artifacts caused by static magnetic field inhomogeneity can be reduced.
In accordance with an eighth aspect, the present invention provides the MR imaging method having the aforementioned configuration, characterized in that: said correction pulse is incorporated into a read axis pulse.
Although the correction pulse may be separately applied, it can be incorporated into a read axis pulse.
In accordance with a ninth aspect, the present invention provides an MRI apparatus comprising a transmit coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a receive coil for receiving an NMR signal, scanning means for driving said transmit coil, gradient coil and receive coil to collect data, and data processing means for arithmetically processing the collected data to produce an image, said MRI apparatus characterized in comprising: phase offset measuring means for adding a crusher for resetting the phase of SE/STE signals to a pulse sequence that is repeated for conducting data collection in an SSFP state, and omitting a phase encoding axis pulse therefrom, collecting phase-offset-measurement data in the SSFP state by repeating the resulting pulse sequence, and measuring the phase offset of the FID signal from the phase-offset-measurement data obtained.
In the MRI apparatus of the ninth aspect, the method of measuring the phase of an FID signal as described regarding the first aspect can be suitably implemented.
In accordance with a tenth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: said crusher for resetting the phase of SE/STE signals is a gradient pulse that is applied to at least one of a phase encoding axis and a read axis at a time after a data collection period.
In the MRI apparatus of the tenth aspect, the method of measuring the phase of an FID signal as described regarding the second aspect can be suitably implemented.
In accordance with an eleventh aspect, the present invention provides an MRI apparatus comprising a transmit coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a receive coil for receiving an NMR signal, scanning means for driving said transmit coil, gradient coil and receive coil to collect data, and data processing means for arithmetically processing the collected data to produce an image, said MRI apparatus characterized in comprising: phase offset measuring means for adding a crusher for resetting the phase of an FID signal to a pulse sequence that is repeated for conducting data collection in an SSFP state, and omitting a phase encoding axis pulse therefrom, collecting phase-offset-measurement data in the SSFP state by repeating the resulting pulse sequence, and measuring the phase offset of the SE/STE signals from the phase-offset-measurement data obtained.
In the MRI apparatus of the eleventh aspect, the method of measuring the phase of SE/STE signals as described regarding the third aspect can be suitably implemented.
In accordance with a twelfth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: said crusher for resetting the phase of an FID signal is a gradient pulse that is applied to at least one of a phase encoding axis and a read axis at a time after an RF pulse and before a data collection period.
In the MRI apparatus of the twelfth aspect, the method of measuring the phase of SE/STE signals as described regarding the fourth aspect can be suitably implemented.
In accordance with a thirteenth aspect, the present invention provides an MRI apparatus comprising a transmit coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a receive coil for receiving an NMR signal, scanning means for driving said transmit coil, gradient coil and receive coil to collect data, and data processing means for arithmetically processing the collected data to produce an image, said MRI apparatus characterized in that: said scanning means adjusts the phase of an RF pulse in a pulse sequence that is repeated for conducting data collection in an SSFP state to correct the phase offsets of an FID signal and SE/STE signals, and collects imaging data in the SSFP state by repeating the resulting pulse sequence.
In the MRI apparatus of the thirteenth aspect, the MR imaging method as described regarding the fifth aspect can be suitably implemented.
In accordance with a fourteenth aspect, the present invention provides an MRI apparatus comprising a transmit coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a receive coil for receiving an NMR signal, scanning means for driving said transmit coil, gradient coil and receive coil to collect data, and data processing means for arithmetically processing the collected data to produce an image, said MRI apparatus characterized in that: said scanning means adds a correction pulse for correcting the phase offsets of an FID signal and SE/STE signals to a pulse sequence that is repeated for conducting data collection in an SSFP state, and collects imaging data in the SSFP state by repeating the resulting pulse sequence.
In the MRI apparatus of the fourteenth aspect, the MR imaging method as described regarding the sixth aspect can be suitably implemented.
In accordance with a fifteenth aspect, the present invention provides an MRI apparatus comprising a transmit coil for transmitting an RF pulse, a gradient coil for applying a gradient magnetic field, a receive coil for receiving an NMR signal, scanning means for driving said transmit coil, gradient coil and receive coil to collect data, and data processing means for arithmetically processing the collected data to produce an image, said MRI apparatus characterized in that: said scanning means adjusts the phase of an RF pulse in a pulse sequence that is repeated for conducting data collection in an SSFP state and adds a correction pulse for correcting the phase offsets of an FID signal and SE/STE signals to correct the phase offsets of the FID signal and SE/STE signals, and collects imaging data in the SSFP state by repeating the resulting pulse sequence.
In the MRI apparatus of the fifteenth aspect, the MR imaging method as described regarding the seventh aspect can be suitably implemented.
In accordance with a sixteenth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: said correction pulse is incorporated into a read axis pulse.
In the MRI apparatus of the sixteenth aspect, the MR imaging method as described regarding the eighth aspect can be suitably implemented.
According to the MR imaging method and MRI apparatus of the present invention, it is possible to obtain an image of good image quality without band artifacts.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.